This invention relates to aluminum production and more particularly, it relates to smelting aluminum in a low temperature electrolytic production cell, for example.
In the present commercial technology Hall-Heroult cell, aluminum is collected in the bottom and is tapped periodically to remove the molten aluminum. In low temperature cells with vertical anodes and cathodes, the removal of molten aluminum has not been without problems. For example, in the low temperature cell where inert anodes and cathodes are used, O2 is produced at the anode and the alumina is reduced to aluminum at the cathode. Because the electrolyte is saturated with alumina, alumina particles are present as a slurry and are very difficult to separate from the molten aluminum. Thus, there is a great need for a process that will separate the aluminum from the alumina particles and electrolyte and remove the aluminum from the cell.
It should be noted that U.S. Pat. Nos. 4,865,701 and 5,006,209, incorporated herein by reference, disclose a low temperature cell for making aluminum from a slurry of alumina present in the electrolyte. The overall reaction in the cell is Al2O3=2Al+3/2 O2 producing O2, as noted, rather than carbon dioxide.
U.S. Pat. No. 5,284,562, incorporated herein by reference, also describes an alumina slurry cell wherein an oxidation resistant, non-consumable anode, for use in the electrolytic reduction of alumina to aluminum, has a composition comprising copper, nickel and iron. The anode is part of an electrolytic reduction cell comprising a vessel having an interior lined with metal which has the same composition as the anode The electrolyte is preferably composed of a eutectic of AlF3 and either (a) NaF or (b) primarily NaF with some of the NaF replaced by an equivalent molar amount of KF or KF and LiF.
U.S. Pat. No. 5,489,320 discloses an aluminum smelting by electrolysis, a double salt of KAlSO4, as a feedstock, is heated with a eutectic electrolyte, such as K2 SO4, at 800° C. for twenty minutes to produce an out-gas of SO3 and a liquid electrolyte of K2SO4 with fine-particles of Al2 O3 in suspension having a mean size of six to eight microns.
U.S. Pat. No. 6,811,676 discloses an electrolytic cell for producing aluminum from alumina having a reservoir for collecting molten aluminum remote from the electrolysis.
U.S. Pat. No. 6,866,768 discloses electrolysis of alumina dissolved in a molten salt electrolyte employing inert anode and cathodes, the anode having a box shape with slots for the cathodes.
U.S. Pat. No. 6,419,812 discloses a method of producing aluminum in an electrolytic cell containing alumina dissolved in an electrolyte. The method comprises the steps of providing a molten salt electrolyte in an electrolytic cell having an anodic liner for containing the electrolyte, the liner having an anodic bottom and walls including at least one end wall extending upwardly from the anodic bottom, the anodic liner being substantially inert with respect to the molten electrolyte. A plurality of non-consumable anodes is provided and disposed vertically in the electrolyte. A plurality of cathodes is disposed vertically in the electrolyte in alternating relationship with the anodes. The anodes are electrically connected to the anodic liner. An electric current is passed through the anodic liner to the anodes, through the electrolyte to the cathodes, and aluminum is deposited on said cathodes. Oxygen bubbles are generated at the anodes and the anodic liner, the bubbles stirring the electrolyte. Molten aluminum is collected from the cathodes into a tubular member positioned underneath the cathodes. The tubular member is in liquid communication with each cathode to collect the molten aluminum therefrom while excluding electrolyte. Molten aluminum is delivered through the tubular member to a molten aluminum reservoir located substantially opposite the anodes and cathodes. The molten aluminum is collected from the cathodes and delivered to the reservoir while avoiding contact of the molten aluminum with the anodic bottom.
In spite of these disclosures, there is still a great need for a cell and method for operating it, which permits the use of a low temperature cell, e.g., in a temperature range of about 700° to 850° C., using a slurry of alumina particles in the electrolyte and recovery of aluminum therefrom without contamination with alumina particles or electrolyte. The present invention provides such a cell and method of operation.